PHOTOCHEMISTRY IN HETEROGENEOUS MEDIA - A FOURIER TRANSFORM-EPR STUDY

Fourier transform-electron paramagnetic resonance (FT-EPR) is a novel spectroscopic technique that is ideally suited for the study of transi ent free radicals generated in photochemical reactions. The salient fe atures of the application of FT-EPR in this field of research are the following. (1) The high spectral resolution of the technique makes it possible to identify free radicals. (2) The time evolution of spectra can be monitored over a time regime extending from nanoseconds to mill iseconds. Hence, spectral data can give rate constants of radical form ation and decay. (3) Signal intensities can reflect chemically induced electron polarization effects that provide information on the mechani sm of radical formation. (4) Measurement of spin-relaxation rates give s an insight into molecular motion. In this paper the application of t he technique in a time-resolved study of photoinduced electron transfe r in micellar solutions and in the pores of silica gel is presented. M easurements on micellar solutions concern electron transfer from photo excited tetrasulfonated zinc tetraphenylporphyrin to duroquinone. The study of silica gel systems focused on electron transfer from zinc tet raphenylporphyrin to benzoquinone. The results of the investigation sh ow that the technique can provide valuable insights into photochemical reactions in heterogeneous media. The time evolution of the spectra p rovides information on the formation and decay of radical pairs and fr ee radicals. The data reflect the spatial distribution of reactants an d products. Linewidths and relaxation data give information on the loc ation and motional freedom of free radicals.